As methods for improving the cryogenic temperature toughness of steel, those that involve refining grain structures and adding alloying elements such as Ni are well known.
The method of refining grain structures, among many existing metal processing methods is known as the only method capable of simultaneously improving strength and toughness. This is due to the fact that when the grain is refined, the dislocation density accumulated at the grain boundary is lowered, and the stress concentration on adjacent grain crystals is reduced to prevent breaking strength from being reached, resulting in good toughness.
However, in typical carbon steel, grain refining able to be obtained through controlled rolling and cooling such as a TMCP is about 5 um, and toughness abruptly decreases at a maximum temperature of about −60° C. or below. Also, even when grain size is reduced to 1 um or below through repeated heat treatments, toughness abruptly decreases at about −100° C. and below, so that brittleness occurs at the cryogenic temperature of about −165° C. in an LNG storage tank. Accordingly, steel that has been used to date to cope with the cryogenic temperature of −165° C. in LNG storage tanks has been obtained through both grain refinement and the addition of Ni or the like to secure cryogenic temperature toughness.
In general, strength is usually increased but toughness is decreased when a substitutional alloying element is added to steel. However, it is shown in documents that the addition of an element such as platinum (Pt), nickel (Ni), ruthenium (Ru), rhodium (Rh), iridium (Ir), or rhenium (Re) actually produces an improvement in toughness. Therefore, while the addition of such an alloying element may be considered, the only commercially available element thereamong is Ni.
The steel that has been used over the preceding several decades as cryogenic steel is steel that contains 9% Ni (hereinafter called “9% Ni steel”). For 9% Ni steel in general, after reheating and quenching (Q), a fine martensite structure is made, and then the martensite structure is softened by tempering (T) and retained austenite is simultaneously precipitated by about 15%. Accordingly, the fine lath of the martensite is restored by tempering and given a fine structure of several hundred nm, and austenite of several tens of nm is produced between laths, so that a fine overall structure of several hundred nm is obtained. In addition, by adding 9% Ni, the steel is provided with improved cryogenic temperature toughness properties. Despite having high strength and good cryogenic temperature toughness, however, the use of 9% Ni steel is limited due to the large amount of relatively high-cost Ni that must be added thereto.
To overcome this limitation, techniques have been developed for using Mn instead of Ni to obtain a similar fine structure. U.S. Pat. No. 4,257,808 discloses a technology in which 5% Mn is added instead of 9% Ni, and the resultant steel is subjected to repeated heat treatments four times in an austenite+ferrite two-phase region temperature range to refine the grain structure, after which tempering is performed to improve cryogenic temperature toughness. Laid-open patent 1997-0043139 discloses a technology which similarly adds 13% Mn and subjects the resultant steel to repeated heat treatment four times in an austenite+ferrite two-phase region temperature range to refine the grain structure in a similar manner, after which tempering is performed in order to improve cryogenic temperature toughness.
Another technology is one in which the existing 9% Ni manufacturing process is retained, the amount of Ni is lowered from 9%, and instead, Mn, Cr, or the like is added. Japanese Patent Application Laid-open No. 2007/080646 is a patent in which the amount of added Ni is 5.5% or greater, and instead, Mn and Cr are added in the amounts of 2.0% and 1.5% or less, respectively.
However, the above patents can only obtain a fine structure when repeated heat treatments are performed four or more times and tempering is then performed, whereupon a steel material may be manufactured having good cryogenic temperature toughness. Therefore, due to the added number of times that a heat treatment is performed over the existing two heat treatments, the drawbacks arise from the added heat treatment costs and the requirement for heat treating equipment.